profile: add profile_finish_ts

"date" field in Bazel json profile is a confusing field.

- It uses Java Date.toString() output which includes local timezone
  that could be hard to parse.

- The name is misleading since users could mistook it for profile
  start time, while it actually means the finish/end time, when
  Bazel calls the writer to serialize profiling data to disk.

Add "profile_finish_ts" which has a clearer name and more consistent
value with the time unit being used in the rest of the JSON
profile(microseconds).

We shall deprecate the "date" field in a separate patch in a major
release.

site/en/advanced/performance/json-trace-profile.md

@@ -168,6 +168,7 @@ Example:
   "otherData": {
     "build_id": "101bff9a-7243-4c1a-8503-9dc6ae4c3b05",
     "date": "Wed Oct 26 08:22:35 CEST 2022",
+    "profile_finish_ts": "1677666095162000",
     "output_base": "/usr/local/google/_bazel_johndoe/573d4be77eaa72b91a3dfaa497bf8cd0"
   },
   "traceEvents": [

src/main/java/com/google/devtools/build/lib/profiler/JsonTraceFileWriter.java

@@ -24,6 +24,7 @@
 import java.io.OutputStreamWriter;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
+import java.time.Instant;
 import java.util.Date;
 import java.util.HashMap;
 import java.util.UUID;
@@ -172,13 +173,15 @@ public void run() {
               // The buffer size of 262144 is chosen at random.
               new OutputStreamWriter(
                   new BufferedOutputStream(outStream, 262144), StandardCharsets.UTF_8))) {
+        var finishDate = Instant.now();
         writer.beginObject();
         writer.name("otherData");
         writer.beginObject();
         writer.name("bazel_version").value(BlazeVersionInfo.instance().getReleaseName());
         writer.name("build_id").value(buildID.toString());
         writer.name("output_base").value(outputBase);
-        writer.name("date").value(new Date().toString());
+        writer.name("date").value(finishDate.toString());
+        writer.name("profile_finish_ts").value(finishDate.getEpochSecond() * 1000);
         writer.endObject();
         writer.name("traceEvents");
         writer.beginArray();